A solder mask is usually employed before soldering the electronic devices to the printed wiring board, and is coated to form a film in all areas except where circuit conductors are soldered. This film is indispensable because it functions not only as an insulating membrane preventing the solder from adhering to necessary areas during the soldering, but also acts as a protective membrane preventing the circuit conductor from being corroded by oxidation after exposure to the air.
Hitherto, the solder mask have been formed on the board by screen printing and cured by ultraviolet rays or heat. However, not only has the printed board been steadily refined, enlarged and made into one-board, thus becoming more advanced with remarkable tempo, but also its mounting method have envolved into a surface mount technology (SMT), and therefore the solder mask is also required to be of higher resolution, accuracy and reliability to match the refined SMT. Various improvements in the solder mask have been proposed.
For example, a solder mask composition comprising bisphenol type epoxy acrylate, sensitizer, epoxy compound and epoxy curing agent, has been disclosed in Japanese Patent application No. 50-144431 and Japanese Patent No. 51-40451. In the case of these solder masks, the unexposed areas are removed and developed by an organic solvent. However, there are the risks of environmental pollution, fire and so on since a large amount of the organic solvents are used in the removal (development) of the unexposed areas.
In order to solve this problem, an alkali developing type photoimageable solder mask capable of development by a dilute alkali aqueous solution is proposed.
As for a photocurable composition capable of alkali development, a material consisting mainly of a reactive product as a base polymer, which is obtained by reacting epoxy resin with unsaturated monocarboxylic acid follow by adding polybasic acid anhydride is disclosed in Japanese Patent Publication No. 56-40329 and 57-45785. In addition, a liquid solder mask composition using a novolak type epoxy resin, which is superior in heat and chemical resistance, can be developed by a dilute alkali aqueous solution and is disclosed in Japanese Patent application No. 61-243869. And a photocurable resin comprising a reaction product (A) an epoxy vinyl ester resin obtained by reacting a cresol novolak epoxy resin and (B)an unsaturated monobasic acid and a polybasic acid anhydride and (C) an alkyl ketene dimer is disclosed in U.S. Pat. No. 4,933,259. And a photopolymerizable compound prepared by reacting a novolak type epoxy compound in successively, an unsaturated monocarboxylic acid dimer and a saturated or unsaturated polybasic acid anhydride is disclosed in U.S. Pat. No. 4,948,700. And a photopolymerizable composition comprising (D) a reaction product obtained by reacting epoxy resin (melting point 65.degree..about.90.degree.) and an .alpha.,.beta.-unsaturated carboxylic acid and then reacting with a polybasic acid anhydride is disclosed in U.S. Pat. No. 5,100,767.
However, the above mentioned solder mask composition have some problems after application. They include developing failure because of the advancement of heat curing electrolytic corrosion and discoloration of the surface of copper foil at the time of predrying depending on the combination of polybasic acid anhydride which is employed in order to make the mask soluble in the alkali developer, epoxy resin, and epoxy resin curing agent.
The composition also have the problem of deteriorating the electrical characteristics of the board due to the influence of carboxylic acid produced by the reaction of polybasic acid anhydride. In addition, their usage is restricted due to the compatibility between the carboxylic acid and epoxy resin, therefore the heat resistance and adhesion become worse.
However, with the recent advances of high density mounting, a reduction of the space between conductors have been required. Several demands have been imposed on electrical insulation between conductors, and excellent dimensional precision has been required of solder resist and the like. While the above mentioned photopolymerizable resinous compositions only have low resolving power to reach 150 .mu.m to 300 .mu.m, therefore the demand of high density mounting printed wiring board such as 80 .mu.m to 150 .mu.m has been unable to be satisfied by the compositions.
The purpose of the present invention is to eliminate the above problems of prior art and to provide a photopolymerizable composition useful as a liquid photoimageable solder mask which has superior ultraviolet curability, heat and chemical resistance, flexibility, electrical characteristics and which is possible to develop by a dilute alkali aqueous solution.
The above-described object is attained with a liquid light-sensitive resinous composition containing as main components (A) a photopolymerizable compound represented by the general formula 1! as follow: (which is a reaction product obtained by reacting epoxy resin. (a) containing at least two terminated epoxy groups and .alpha.,.beta.-unsaturated carboxylic acid (b) containing at least three vinyl groups represented by the general formula II! and an .alpha.,.beta.-unsaturated carboxylic acid (c) and then reacting with polybasic acid anhydride (d) and then reacting with a monoepoxide (e) containing one vinyl group); ##STR1## (B) a photocurable monomer containing carboxylic acid represented by the general formula II!: ##STR2## (C) a photocurable monomer containing at least one vinyl group; (D) a photopolymerization initator;
(E) a curing agent which makes prepolymer (A) undergo thermal reaction; PA1 (F) organic solvents.